this is purely anectdotal and obviously i am not a pro but this has been my take on Israel haters
1. Knee jerk liberalism blames the US for everything and Israel an extension of the US so therefore they get blamed for anything that happens in that region.
2. knee jerk liberals rally for the “little guy” at least in theory and see the Palestinians as victims of the “man” being Israel.
3. some people have ties to say Lebanon or Syria and therefore feel wronged by Israel for one thing or another.
4. Israel has made its share of mistakes including opening fire on a US naval ship (i think it was called the USS liberty but not sure)
5. basic anti-semitism.
now if you are looking at European Israel haters the reasons are a little more personal
1. they have a significant Arab/Muslim minority so they like to placate them
2. they are also on the liberal side so have a knee jerk reaction for the perceived “little guy”
3. they are a heck of a lot closer to the action so fear terror reprisals if they ally too close with Israel.
4. they have big oil contracts with Iran and again fear pissing them off
5. still some deep rooted anti-semitism in Europe
well those are my observations anyway
There’s one more point on the US side. A Libertarian friend of mine(and Ron Paul supporter) is also anti-Israel. He claims they secretly run our government, financial institutions etc.